The present invention concerns a process for producing methane by reacting hydrogen with carbon oxides, particularly carbon monoxide.
It is known that one of the main difficulties in the catalytic synthesis of methane according to the equation: EQU 3H.sub.2 +CO.fwdarw.CH.sub.4 +H.sub.2 O
is a technological one and is related to the very high exothermicity of the reaction, so that for a mixture of H.sub.2 and CO in stoichiometrical proportions, the temperature increase under adiabatic conditions is from 18.degree. C. to 24.degree. C. for each percent of conversion. The potential temperature increase for an assumed total conversion would thus be compared between 1800.degree. and 2400.degree. C.
Different techniques have been proposed to solve this problem:
1. The recycling of a substantial fraction of the gas in order to decrease the carbon oxide concentration; the heat evolved is then removed outside of the reactor, PA1 2. The use of several reactors in series; the conversion rate in each reactor is limited by the selection of the operating conditions and the heat is removed between each reactor, PA1 3. The use inside the reactor itself of a tubular system through which a cooling fluid is circulated. PA1 4. A technique proposed in the U. S. Pat. No. . 3,930,812, consists of passing liquid water through a methanation catalyst, simultaneously with the synthesis gas, the partial or total vaporization of said water making it possible to remove the reaction heat.
However, it is well known that most of the methanation catalysts, which are very active in gaseous phase or in the presence of a hydrocarbon liquid phase, generally quickly deactivate in the presence of liquid water. In the case of catalysts in bulk this effect is attributed to a sintering of the active phase. As concerns the catalysts formed of an active phase deposited and dispersed on an inorganic carrier, the deactivation may be attributed to the instability of the carriers commonly used in the presence of liquid water under the reaction conditions, i.e. at temperatures from 200.degree. to 350.degree. C. Thus, for example, a .gamma. alumina of high surface recrystallises to .alpha. alumina of low surface when it is heated for several days in liquid water.